1. Field of the Invention
The present invention relates to motors, in particular low cost motors, and to improvements to motors usable as general-purpose industrial motors, and furthermore to motors useful as high rotational speed motors in which centrifugal force becoming a problem in terms of rotor strength.
2. Description of the Related Art
When it is necessary to rotate a motor at high speeds, such as the high-speed main shaft of a machine tool in a machining center, the motor's rotor with a diameter of about 100 mm must reach speeds of at least 30,000 rpm. Although induction motors are used for this sort of application, the slots of the rotor are not opened and are often closed to withstand the centrifugal force, and the rotor coil ends are also often reinforced. In any case, the cost increases and a reinforced construction that somewhat sacrifices the motor characteristics is often employed.
Switched reluctance motors, which have high rotor strength, have been researched and some have been put into actual use. A typical example of an actual motor is shown in FIG. 10. A rotor 2 is a simple silicon steel plate and extremely strong so as to be suitable for high-speed rotation.
A stator 1 has six salient poles 20 and the width of each salient pole 20 is approximately 30 degrees when converted to the rotor rotational angle. Each salient pole 20 is wound with windings TA1, TA2, TB1, TB2, TC1, TC2, TD1, TD2, TE1, TE2, TF1, and TF2. The rotor 2 has four salient poles 21 and the width of each salient pole 21 is approximately 30 degrees when converted to the rotor rotational angle.
The operation of the switched reluctance motor is described next. For example, when generating a rotational torque in the counterclockwise direction in the state of FIG. 10, current is passed through the windings indicated by TC1 and TC2, and TF1 and TF2 so that the salient poles 21 of rotor 2 are attracted to generate a rotational torque. At this time, the current flowing through the windings indicated by TC1 and TC2 and the current flowing through the windings indicated by TF1 and TF2 have opposite directions, and the currents flow so that the generated magnetic flux passes through the rotor 2. Furthermore, while the rotor 2 rotates in the counterclockwise direction, the rotational torque is not generated when the salient poles 21 of rotor 2 reach the position of the stator pole wound with windings TC1 and TC2. At this time, since the adjacent rotor salient pole in the counterclockwise direction approaches the stator salient pole wound with windings indicated by TE1 and TE2, setting the current in the windings TC1 and TC2 to zero, and at the same time passing current in the windings indicated by TE1 and TE2 and the windings indicated by TB1 and TB2 causes a rotational torque to be generated in the counterclockwise direction. In this manner, passing an appropriate intermittent current in sequence to each stator winding enables a continuous rotational torque to be generated. Simultaneously, when generating a rotational torque in the clockwise direction in the state of FIG. 10, current is passed through the windings indicated by TB1 and TB2 so that the salient poles of rotor 2 are attracted to generate a rotational torque.
The generated torque is related to the current in the windings and the relative positions of the stator 1 and rotor 2, and in theory is unrelated to the rotational speed of the rotor.
Characteristics of this switched reluctance motor include a low fabrication cost due to a simple motor construction and a simple winding structure of the stator windings, a relatively short-motor length because the coil ends of the stator windings can be shortened, a durable rotor making it physically possible for high-speed rotation, and a drive circuit that can be simplified since the drive algorithm is simple and only one direction of current is sufficient.
On the other hand, the switched reluctance motor also has a number of shortcomings. When the control algorithm to even the relationship of the supplied electrical energy, the magnetic energy stored in the motor, and the mechanical output energy has not been established, the result is a large torque ripple. One method that has been proposed to solve this is to compensate for the current so as to compensate for torque ripple, thus reducing torque ripple. However, this method introduces other problems, such as the requirement of a complex control method. In addition to torque ripple, the intermittent torque generated by each salient pole also affects in terms of motor strength the stator deformation, and vibration and noise when the motor is driven are large. Furthermore, other problems include the requirement of high-speed current control and the requirement of extremely high-speed current switching for the high-speed rotation of the four-pole motor compared to that for the two-pole motor. Furthermore, there is the problem of the power factor since it is necessary to frequently perform the supply and regeneration of magnetic energy in the motor.